1. Field of the Invention
The present invention is related to a thin film transistor and a method for manufacturing the thin film transistor, the transistor being able to be used in a driving device of various image display devices, a logic device of various logic circuits or the like.
2. Description of the Related Art
With the remarkable development of information technology, presently, information is frequently sent or received using a mobile information terminal, a notebook computer and the like. It is well-known that, in the near future, the ubiquitous society will be realized in which information is sent or received from any place. In such a society, a thinner and lighter information terminal is desired.
Currently, various image display devices and various logic circuits are incorporated in an information terminal. A thin film transistor is used for most of their driving devices and logic devices.
Currently, the main stream of a semiconductor material is silicon type (Si type). However, the research and development of the transistor using an organic semiconductor (the organic transistor) has become active in light of improving flexibility, reducing weight and cost. Generally, in the case of using an organic semiconductor, because it is possible to process in a wet state, there are some advantages such as increasing the size as much as possible, being able to adopt a printing method and being able to use a plastic substrate. (See non-patent document 1.)
In addition, the application field of the organic semiconductor is wide, and is not limited to the above-mentioned thin and light flexible display. The application of the organic semiconductor to RFID (radio frequency identification) tag or a sensor is expected. In this way, the research and development of the organic transistor is necessary for the ubiquitous society.
For these reasons, an organic semiconductor which is formed by a printing method is currently attracting attention. However, currently, the mobility of a carrier of a field-effect transistor using an organic semiconductor is low, for example 10−2-10−4 cm2/Vs order and the organic semiconductor has not reached a sufficient level for practical use. Therefore, development of the organic semiconductor in various points such as an organic semiconductor material, an electrode material, a device constitution and a manufacturing process is rapidly needed.
Among these problems, a problem in an interface between a semiconductor and an electrode is important. The characteristics of a transistor largely change depending on whether a carrier can be efficiently injected from an electrode to a semiconductor or not. At present, most compounds used for an organic semiconductor are p-type semiconductors in which the carrier is a hole. Therefore, it is desirable that the work function of an electrode be large in order to allow an ohmic contact. Further, it is desirable that the work function of an electrode be larger than the work function of a semiconductor. The work function of a p-type organic semiconductor is about 5.0-5.5 eV. It has been found that the use of a metal material such as platinum (5.65 eV) and gold (5.1 eV), and a conductive polymer material such as poly (ethylenedioxy thiophen)/polystyrene sulfonate (PEDOT/PSS) (5.0 eV) and polyaniline (4.9 eV), as an electrode material having a high work function, can allow the efficiency of carrier injection to be increased.
In one example, a thin film transistor using gold or platinum having a high work function for a source electrode and a drain electrode is manufactured (See patent document 1.)
However, the variety of metal materials having a high work function is few and the material is rare. Therefore, it is difficult to manufacture an organic transistor having an efficient carrier injection that can be manufactured from a wide range of electrode materials.                [Patent document 1] JP-A-2000-174277        [Non-patent document 1] Science Vol. 265, 1684 (1994)        